# Swap Fees

A swap fee is charged for each swap, as well as on the non-proportional amounts in add/remove liquidity operations. The swap fee is always charged on the amount in:

• EXACT_IN, on the given amount (see Vault.solopen in new window):

// Round up to avoid losses during precision loss.
locals.totalSwapFeeAmountScaled18 = poolSwapParams.amountGivenScaled18.mulUp(swapState.swapFeePercentage);
poolSwapParams.amountGivenScaled18 -= locals.totalSwapFeeAmountScaled18;

• EXACT_OUT, the calculated amount (see Vault.solopen in new window)):

// To ensure symmetry with EXACT_IN, the swap fee used by ExactOut is
// `amountCalculated * fee% / (100% - fee%)`. Add it to the calculated amountIn. Round up to avoid losing
// value due to precision loss. Note that if the `swapFeePercentage` were 100% here, this would revert with
// division by zero. We protect against this by ensuring in PoolConfigLib and HooksConfigLib that all swap
// fees (static, dynamic, pool creator, and aggregate) are less than 100%.
locals.totalSwapFeeAmountScaled18 = amountCalculatedScaled18.mulDivUp(
    swapState.swapFeePercentage,
    swapState.swapFeePercentage.complement()
);

Swap fees come in two different forms for V3 pools:

• Static Swap Fee:
  • Initially set as part of the pool registration.
  • Authorized addresses can then change the value by invoking the vault.setStaticSwapFeePercentage(address pool, uint256 swapFeePercentage) function.
  • If the staticSwapFeePercentage is changed, it will emit an event: SwapFeePercentageChanged(pool, swapFeePercentage);
  • Note - setStaticSwapFeePercentage can also be called as part of a regular hook
• Dynamic Swap Fee Using Hooks:
  • Using Hooks a pool can be set up to use dynamic swap fees.
  • The Vault uses the onComputeDynamicSwapFeePercentage() hook to fetch the dynamic swap fee. This function can implement arbitrary logic.
  • Even when a pool is set to use dynamic swap fees, it still maintains a static swap fee. However, this static fee is not utilized.

The pseudo logic to determine how swap fee is calculated looks like:

swapFeePercentage =
     Pool has DynamicSwapFee => call DynamicSwapFeeHook in the pool
     else => load static Swap fee percentage from Vault